WU Zhen, CHEN Weixin, TAO Xueyu, ZHENG Ming, FAN Heliang, LIU Zhangsheng, GUO Litong
(School of Materials Science and Physics, China University of Mining and Technology, Xuzhou 221116, Jiangsu, China)
Extended Abstract:[Background and purpose] Although carbon materials have excellent high-temperature structural properties, such as high melting point, low density and low coefficient of thermal expansion, it is challenging to avoid high-temperature oxidation. In this article, HfB2-TaB2-ZrB2-SiC-MoSi2 composite coating on the surface of carbon materials was prepared by using laser cladding method, providing wide temperature range protection for the carbon material substrate.[Methods] The laser cladding was carried out with a CO2 laser (JQ1390), with powers of 50–70 W, spot diameters of 0.1–0.2 mm and scanning rates of 10–20 mm·s−1. The same laser was used to test the lasing resistance. A scratch adhesion tester (WS-2005) was used to measure the adhesion between the coating and carbon matrix. Phase composition of the samples after laser cladding and laser ablation was analyzed by using X-ray diffractometer (Bruker D8-ADVANCE). Morphology and element distribution of the coatings were characterized by using scanning electron microscope (SEM, SU3500) and energy dispersive spectrometer (SU3500). The isothermal oxidation experiment in air was carried out in an air furnace.[Results] The coating had strong adhesion to the carbon matrix, with a scratch adhesion of 16.8 N, corresponding to scratch strength of 134.06 MPa. The cross-section of the coating had no obvious defects, such as penetrating cracks and pores. There was an oxide glass layer of about 6 μm on the upper part of the coating, while the unoxidized area with a thickness of about 18 μm inside was divided into a fine grain area in the upper half and a coarse grain area in the lower half. During the laser cladding process, the cooling rate in the upper half is faster and the grains are finer. The overall grain size distribution of the coating was relatively uniform and dense, with no obvious cracks or pores, which could provide good protection for the substrate.After laser line ablation, the surface of the coating became smoother and free of defects, which indicated the strong laser ablation resistance. After laser line ablations for five times, the adhesion of the coating scratch remained at 14.2 N, corresponding to a scratch strength of 113.1 MPa. The coating sample formed a dense Hf-Zr-Ta-Si-O composite oxide layer on the surface during isothermal oxidation at 1500 ℃, enhancing its high-temperature stability. The oxide layer on the surface of the coating was dense, complete and free of obvious defects, which exhibited strong overall oxidation resistance and laser ablation resistance. Replacing some Si with MoSi2 can further enhance the high-temperature oxidation resistance and laser ablation resistance.Si element is widely distributed in the coating, which indicated that it could act as a binder after melting, filling the grain gaps and forming a dense coating structure. The O element is mainly enriched on the surface of the coating, while there is almost no O element inside the coating, which indicated that there was no significant oxidation inside the coating, while it was still mainly composed of the raw material components, such as ZrB2, HfB2 and TaB2. There are no obvious cracks between the coating and the carbon substrate, due to the formation of a tightly bonded transition zone. The 11# coating exhibits excellent scratch bonding strength with the substrate.The scratch adhesions of A1 coating after laser cladding and five laser line ablations were 15.51 N and 14.20 N, respectively, corresponding to scratch strengths of 123.49 MPa and 113.06 MPa. The scratch adhesions of A2 coating were 9.67 N and 18.76 N, corresponding to scratch strengths of 76.99 MPa and 149.37 MPa, respectively. The coating maintained a high overall scratch strength before and after laser ablation. The increase in scratch strength of A2 coated samples after laser ablation is mainly attributed to the smoother surface after laser ablation. The scratch strength of A1 coated samples slightly decreased after laser ablation, due to the formation of holes on the surface.[Conclusions] HfB2-TaB2-ZrB2-SiC-MoSi2 composite coating was deposited on the surface of carbon substrate by using laser cladding to improve the high temperature oxidation resistance of carbon materials. The weight of all samples rapidly increased after the first 10 mins of oxidation and then gradually stabilized. The borides and carbides in the coating gradually transformed into oxides, such as HfO2, Ta2O5, ZrO2, B2O3, and SiO2 during oxidation. Then, a dense B2O3-SiO2 glass layer was formed on the surface. Meanwhile, ZrSiO4 and HfSiO4 phases were formed on the surface, further improving the stability of the glass layer, preventing further infiltration of oxygen and hence increasing the protection ability of the matrix.
Key words: laser cladding; UHTCs; anti-oxidation coating; laser ablation; carbon material